Electrophotographic image forming methods include forming an electrostatic latent image on an image bearer such as a photoconductive material, transferring a charged toner thereto to form a visible image (toner image), transferring the toner image onto a recording medium such as paper, and fixing the toner image thereon to form a final output image. Recently, electrophotographic copiers and printers are rapidly developing from monochrome to full-color, and full-color markets are expanding.
The electrophotographic image forming methods typically include overlaying three primary color toners, i.e., yellow, magenta, and cyan toners, or four color toners including the previous three and black toner, to reproduce all colors. Therefore, to produce a sharp full-color image having good color reproducibility, the surface of a fixed toner image has to be smooth to reduce light scattering. This is why many conventional full-color copiers produce images having medium to high glossiness of from 10 to 50%.
As a method of fixing a dry toner image on a recording medium, contact heat fixing methods using a heated roller or a heated belt having a smooth surface are typically used. Although the methods have high heat efficiency and are capable of fixing at high speed and imparting gloss and transparency to color toners, offset problems do occur, wherein a part of a toner image adheres to a fixing member and transfers to another image occur during separation of the fixing member from the melted toner image after pressing the surface of the fixing member.
For the purpose of preventing offset problems, the surface of the fixing member has typically been coated with silicone rubber or a fluorine-containing resin. Further, a releasing agent in the form of an oil such as silicone oil is applied to the surface of the fixing member. Although this approach is quite effectively used to prevent offset problems, a release oil applicator is required and the resultant fixer becomes larger. Therefore, as an alternative, there are oilless arrangements using a monochrome toner having high viscoelasticity when melted so as not to break down internally, wherein the molecular weight distribution of a binder resin of the toner is controlled, and further including a release agent such as wax, or systems that apply only a small amount of oil.
In addition, even full-color image forming apparatuses are becoming oilless as well for the purpose of being downsized and simplified. However, as mentioned above, to improve color reproducibility of a color toner, the color toner needs to have lower viscoelasticity because the fixed color toner image is required to have a smooth surface. Therefore, the color toner has offset problems more often than the monochrome toner does, making it more difficult to make a fixer oilless or use only a small amount of oil. In addition, a toner including a release agent has higher adherence to an image bearer and lower transferability to a transfer paper. Further, the release agent therein contaminates friction-charged members such as a carrier and lowers the chargeability thereof, resulting in deterioration of durability of the toner.
In proportion to increase of demand for faster and prettier image formation, a developer including a carrier and a toner receives significantly more stress. Even the carrier conventionally having a long life is becoming more difficult to have a sufficient life.
Further, the carrier are having smaller particle diameter to produce higher quality images, which is largely effective for producing high-definition images but also causes many adverse effects. One of them is deterioration of storage stability in an environment of high temperature and high humidity, e.g., a developer is solidified, i.e., a carrier and a toner, carriers themselves, or toners themselves adhere to each other to form an agglomerate. When a brand new developer is set in a developing unit, ordinarily it falls therein with ease. However, a solidified developer does not, and even when fed therein, the solidified developer remaining therein is stuck with a regulation plate regulating an amount of the developer on a developing roller, resulting in production of abnormal images. When solidified in the developing unit or placed therein from the beginning without a case containing the developer and too solidified to break with an agitation blade therein, abnormal images are produced or the unit is not drivable.
This is partly because both of the carrier and the toner increase their specific surface area with smaller particle diameters, and partly because of low-temperature fixability and oilless. In addition, the developing unit becomes smaller with smaller machine and the developer is difficult to fall, and problems tend to occur when the developer is solidified. With variation of the market, a guaranteed environment needs to be wider and a developer is actually required to have more strict quality.
In order to solve this problem, Japanese published unexamined application No. 2006-154453 discloses a method of increasing a temperature of burning a resin-coated carrier in an electric oven. However, the resin tends to become black with high temperature, resulting in production of contaminated color images.
Thus, the solidification of the developer is thought a serious problem from now. Toner solidifications are conventionally disclosed in Japanese published examined application No. 2-51177 and Japanese published unexamined applications Nos. 9-304959, 9-329910 and 2001-312093. However, they are all about toners only, not about solidification of a developer including a toner and a carrier.
Japanese published unexamined applications Nos. 2007-102159 and 2008-70837 disclose a carrier for electrophotography including particles each having a particle diameter larger than the thickness of the coated layer of the carrier. Further, Japanese published unexamined application No. 2007-286078 discloses a developing method feeding the toner and carrier to an image developer while discharging an extra developer in the image developer.
Demand for faster and prettier image formation is increasing, and the developing roller rotates faster and a centrifugal force applied to the developer is increasing as well. Further, each of the carrier and the toner is required to have a smaller particle diameter to produce higher quality images.
Therefore, the particle diameter does not affect the centrifugal force if magnetizations of the carriers are same (the magnetization per weight is same). However, the smaller the particle diameters of the toner and the carrier, the specific surface area per weight increases, resulting in a charge quantity larger than the magnetization. Namely, the carrier is electrically attracted to a photoreceptor to adhere thereto.
However, effects of the above-mentioned conventional arts against initial carrier adherence, solidification, color contamination and abrasion are insufficient.
Because of these reasons, a need exists for a method of preparing a carrier without initial adherence, and having good anti-solidification, anti-color contamination and abrasion resistance.